Sodium-Vanadium Bronze Na9V14O35: An Electrode Material for Na-Ion Batteries

Na9V14O35 (η-NaxV2O5) has been synthesized via solid-state reaction in an evacuated sealed silica ampoule and tested as electroactive material for Na-ion batteries. According to powder X-ray diffraction, electron diffraction and atomic resolution scanning transmission electron microscopy, Na9V14O35 adopts a monoclinic structure consisting of layers of corner- and edge-sharing VO5 tetragonal pyramids and VO4 tetrahedra with Na cations positioned between the layers, and can be considered as sodium vanadium(IV,V) oxovanadate Na9V104.1+O19(V5+O4)4. Behavior of Na9V14O35 as a positive and negative electrode in Na half-cells was investigated by galvanostatic cycling against metallic Na, synchrotron powder X-ray diffraction and electron energy loss spectroscopy. Being charged to 4.6 V vs. Na+/Na, almost 3 Na can be extracted per Na9V14O35 formula, resulting in electrochemical capacity of ~60 mAh g−1. Upon discharge below 1 V, Na9V14O35 uptakes sodium up to Na:V = 1:1 ratio that is accompanied by drastic elongation of the separation between the layers of the VO4 tetrahedra and VO5 tetragonal pyramids and volume increase of about 31%. Below 0.25 V, the ordered layered Na9V14O35 structure transforms into a rock-salt type disordered structure and ultimately into amorphous products of a conversion reaction at 0.1 V. The discharge capacity of 490 mAh g−1 delivered at first cycle due to the conversion reaction fades with the number of charge-discharge cycles.


Introduction
A commercial application of rechargeable sodium-ion batteries (SIBs) would bring substantial alleviation and expansion of the existing energy storage market, which is mainly based on the Li-ion battery (LIB) technology. In terms of material abundance, SIBs appear to be the cheaper alternative to LIBs that enables their usage in high-scale energy storage, for instance, in smart-grid applications. The positive electrode (cathode) materials, especially the layered transition metal oxides, are an intensively studied topic in the field of SIBs, mostly because of the fact that the overall battery energy and power density are primarily limited by the cathode material. Since the ionic volume of sodium is about 70% larger than that of lithium, the structural chemistry of the Na-ion (de)intercalation systems is more complicated compared to the Li-based ones as the size difference of Na + and transition metal cations M 2+ or M 3+ is quite large, demanding higher flexibility of the hosting frameworks. State-of-the art cathode materials for SIBs belong to two large groups: layered and 3-dimensional 3d and 4d metal oxides and polyanion structures (phosphates, sulphates, etc.) [1,2].
Vanadium pentoxide V 2 O 5 , belonging to the family of 2D oxides, was studied as an insertion structure for both sodium and lithium ions [3]. A family of sodium-vanadium bronzes with the general formula Na x V 2 O 5 (0 < x ≤ 2) with mixed valence of the vanadium ions between V 4+ and V 5+ , unlike the bronzes of other transition metals, comprises a wide variety of structures termed α-, β-, γ-, δ-, τ-, α'-, η-, κ and χ [4][5][6][7][8][9][10]. The general structure motive for the Na x V 2 O 5 bronzes is adopted from the layered structure of V 2 O 5 , which is built of edge-and vertex-sharing VO 5 square pyramids (Figure 1a), though the crystal structures of the bronzes are exceptionally flexible as exemplified with the layered (α-phase) and tunnel (β-phase) materials [11,12]. Among sodium-based vanadium bronzes, monoclinic β-Na 0.33 V 2 O 5 has gained much attention because of tunnel structure, adopting three different Na intercalation sites and ensuring good structural reversibility even upon deep charge/discharge. Numerous attempts in preparation of nanostructured β-Na 0.33 V 2 O 5 resulted in impressive discharge capacity above 300 mAh g −1 in the Li-ion cells [13][14][15][16]. The cycling performance is highly dependent on the particle's morphology, potential window and current density, and the best compromise between these electrochemical characteristics seems to be reached for the micro-rod β-Na 0.33 V 2 O 5 material showing 297 mAh g −1 discharge capacity at low current density (1.5-4.0 V vs. Li + /Li), which is retained with a high efficiency after at least 50 cycles [13].
Molecules 2022, 26, x FOR PEER REVIEW 2 of 12 bronzes with the general formula NaxV2O5 (0 < x ≤ 2) with mixed valence of the vanadium ions between V 4+ and V 5+ , unlike the bronzes of other transition metals, comprises a wide variety of structures termed α-, β-, γ-, δ-, τ-α'-, η-, κ and χ [4][5][6][7][8][9][10]. The general structure motive for the NaxV2O5 bronzes is adopted from the layered structure of V2O5, which is built of edge-and vertex-sharing VO5 square pyramids (Figure 1a), though the crystal structures of the bronzes are exceptionally flexible as exemplified with the layered (α-phase) and tunnel (β-phase) materials [11,12]. Among sodium-based vanadium bronzes, monoclinic β-Na0.33V2O5 has gained much attention because of tunnel structure, adopting three different Na intercalation sites and ensuring good structural reversibility even upon deep charge/discharge. Numerous attempts in preparation of nanostructured β-Na0.33V2O5 resulted in impressive discharge capacity above 300 mAh g −1 in the Li-ion cells [13][14][15][16]. The cycling performance is highly dependent on the particle's morphology, potential window and current density, and the best compromise between these electrochemical characteristics seems to be reached for the micro-rod β-Na0.33V2O5 material showing 297 mAh g −1 discharge capacity at low current density (1.5-4.0 V vs. Li + /Li), which is retained with a high efficiency after at least 50 cycles [13]. The α-V2O5 matrix could adopt various phase transformations during Li + (de)intercalation. For example, the reduction of V2O5 in an anodic range below 1.9 V [17] results in formation of the Li3V2O5 phase with a disordered rock-salt structure, which can be reversibly cycled between 0.01 V and 2 V with a specific discharge capacity of 266 mAh g −1 (current density 0.1 A g −1 ) [18]. Impressive cycling performance of Li3V2O5 is preserved even at higher cycling rates, demonstrating the discharge capacity of 200 mAh g −1 after 1000 cycles at 1 A g −1 .
Information on electrochemical behavior of vanadium bronzes or V2O5 in Na cells is still limited. γ-NaxV2O5 (x = 0.96; 0.97) synthesized by electrochemical reduction of γ'-V2O5 exhibits an orthorhombic layered structure (S.G. Pnma) related to the parent structure of V2O5, and shows specific capacities between 80-125 mAh g −1 in the one-step sodium-extraction-insertion process at 3.3-3.4 V vs. Na + /Na [7,9]. Muller-Bouvet et al. studied the electrochemical behavior of α'-NaV2O5, which was electrochemically formed during discharge of V2O5 in Na cell in the 3.0-1.6 V potential range. The α'-NaV2O5 bronze with an orthorhombic structure, which delivers a specific capacity of 120 mAh g −1 at 0.1 mA cm −2 current density, is also suitable for reversible sodium intercalation [19]. High discharge capacity of 250 mAh g −1 retained with 88% efficiency after 320 cycles at 20 mA g −1 (3.8-1.5 V vs. Na + /Na) was reported for so-called "bilayered" V2O5 with short-range ordering in the crystal structure, though no structural data were provided for the Na-containing phases formed during the reversible (de)intercalation process [20]. Nanostructured Na0.33V2O5 tested as an electrode material within the potential window of The α-V 2 O 5 matrix could adopt various phase transformations during Li + (de)intercalation. For example, the reduction of V 2 O 5 in an anodic range below 1.9 V [17] results in formation of the Li 3 V 2 O 5 phase with a disordered rock-salt structure, which can be reversibly cycled between 0.01 V and 2 V with a specific discharge capacity of 266 mAh g −1 (current density 0.1 A g −1 ) [18]. Impressive cycling performance of Li 3 V 2 O 5 is preserved even at higher cycling rates, demonstrating the discharge capacity of 200 mAh g −1 after 1000 cycles at 1 A g −1 .
Information on electrochemical behavior of vanadium bronzes or V 2 O 5 in Na cells is still limited. γ-Na x V 2 O 5 (x = 0.96; 0.97) synthesized by electrochemical reduction of γ'-V 2 O 5 exhibits an orthorhombic layered structure (S.G. Pnma) related to the parent structure of V 2 O 5 , and shows specific capacities between 80-125 mAh g −1 in the one-step sodium-extraction-insertion process at 3.3-3.4 V vs. Na + /Na [7,9]. Muller-Bouvet et al. studied the electrochemical behavior of α'-NaV 2 O 5 , which was electrochemically formed during discharge of V 2 O 5 in Na cell in the 3.0-1.6 V potential range. The α'-NaV 2 O 5 bronze with an orthorhombic structure, which delivers a specific capacity of 120 mAh g −1 at 0.1 mA cm −2 current density, is also suitable for reversible sodium intercalation [19]. High discharge capacity of 250 mAh g −1 retained with 88% efficiency after 320 cycles at 20 mA g −1 (3.8-1.5 V vs. Na + /Na) was reported for so-called "bilayered" V 2 O 5 with short-range ordering in the crystal structure, though no structural data were provided for the Na-containing phases formed during the reversible (de)intercalation process [20]. Nanostructured Na 0.33 V 2 O 5 tested as an electrode material within the potential window of 1.5-4.0 V vs. Na + /Na demonstrated capacity of 130 mAh g −1 at the first discharge, and it showed gradual decay up to 90 mAh g −1 after 50 cycles at a 50 mA g −1 current density [21]. Inspired by high specific capacities and long cycling performance of vanadium bronzes, on the one hand, and the lack of a comprehensive study of vanadium bronzes in Na cells, on the other hand, we tailored this study to investigate the η-Na x V 2 O 5 (x~1.29) bronze as the host structure for (de)intercalation of Na cations. η-Na x V 2 O 5 or Na 9 V 14 O 35 crystallizes in the monoclinic lattice with the space group P2/c [22,23] and adopts a crystal structure built of (010) layers formed by VO 5 (V 4+ ) square pyramids and VO 4 (V 5+ ) tetrahedra, with the sodium atoms embedded between the layers (Figure 1b). Similarly to the mixedvalence sulfates, phosphates and other polyanion structures, Na 9 V 14 O 35 can be considered as sodium vanadium(IV,V) oxovanadate Na 9 V 10 4.1+ O 19 (V 5+ O 4 ) 4 . Theoretical capacity of η-Na x V 2 O 5 in case of extraction of all sodium atoms can be estimated as~163 mAh g −1 that, being complemented with possible electrochemical activity in the anodic area, makes this material of potential interest as a new intercalation system for Na ions. In this paper, we report on synthesis, multidisciplinary study of Na 9 V 14 O 35 as both positive and negative electrode materials in Na half-cells and evolution of the crystal structure upon gradual uptake of sodium atoms in the low-voltage range.

Compositional and Crystallographic Characterization
The PXRD pattern of Na 9 V 14 Figure S1, Tables 1, S1 and S2) was performed using a structure model reported by Isobe et al. [23]. The [010] HAADF-STEM image of Na 9 V 14 O 35 demonstrates a well-ordered structure, where vanadium atomic columns appear as prominent bright dots, while the sodium columns are visible as faint dots, due to the difference in atomic numbers of V and Na (Figure 2b).
Molecules 2022, 26, x FOR PEER REVIEW 2 of 12 1.5-4.0 V vs. Na + /Na demonstrated capacity of 130 mAh g −1 at the first discharge, and it showed gradual decay up to 90 mAh g −1 after 50 cycles at a 50 mA g −1 current density [21]. Inspired by high specific capacities and long cycling performance of vanadium bronzes, on the one hand, and the lack of a comprehensive study of vanadium bronzes in Na cells, on the other hand, we tailored this study to investigate the η-NaxV2O5 (x ~ 1.29) bronze as the host structure for (de)intercalation of Na cations. η-NaxV2O5 or Na9V14O35 crystallizes in the monoclinic lattice with the space group P2/c [22,23] and adopts a crystal structure built of (010) layers formed by VO5 (V 4+ ) square pyramids and VO4 (V 5+ ) tetrahedra, with the sodium atoms embedded between the layers (Figure 1b). Similarly to the mixed-valence sulfates, phosphates and other polyanion structures, Na9V14O35 can be considered as sodium vanadium(IV,V) oxovanadate Na9V10 4.1+ O19(V 5+ O4)4. Theoretical capacity of η-NaxV2O5 in case of extraction of all sodium atoms can be estimated as ~163 mAh g −1 that, being complemented with possible electrochemical activity in the anodic area, makes this material of potential interest as a new intercalation system for Na ions. In this paper, we report on synthesis, multidisciplinary study of Na9V14O35 as both positive and negative electrode materials in Na half-cells and evolution of the crystal structure upon gradual uptake of sodium atoms in the low-voltage range.

Compositional and Crystallographic Characterization
The PXRD pattern of Na9V14O35 was indexed with a monoclinic unit cell with parameters a = 15.1990 (2) Table 1, S1 and S2) was performed using a structure model reported by Isobe et al. [23]. The [010] HAADF-STEM image of Na9V14O35 demonstrates a well-ordered structure, where vanadium atomic columns appear as prominent bright dots, while the sodium columns are visible as faint dots, due to the difference in atomic numbers of V and Na ( Figure 2b).   The synthesized polycrystalline Na9V14O35 powder consists of thin platelets stacked into micron-sized agglomerates (Figure 3). The observed morphology corroborates with the strong preferred orientation effect visible as drastic difference in reflection intensities in the PXRD diffraction patterns collected with the diffractometers with the transmission and reflection (Bragg-Brentano) geometry ( Figure 4) due to different angular relations between the preferred orientation axis dt and reflection vector Hhkl. The observed intensity variation indicates the preferred orientation of platelets perpendicular to the b axis. The EDX compositional maps of Na9V14O35 demonstrate homogeneous distribution of sodium and vanadium cations ( Figure S2 of SI).

Electrochemical Characterization
As mentioned in Section 4, the active material was ball-milled to reduce the particle size. However, the discharge capacity in the first discharge cycle was only~32 mAh g −1 for the 1.5-4.8 V potential window ( Figure S3). The long plateau during the first charge above~4.6 V is tentatively attributed to the electrolyte oxidation, thus further tests were limited by the 4.6 V threshold.
Galvanostatic cycling in the 0.1-4.6 V potential window was performed at C/20, C/10 and C/5 rates (Figure 5a-c). The shape of galvanostatic curves looks quite similar for the C/20 and C/10 rates and can be characterized by a low charge capacity in the first cycle (~50-60 mAh g −1 ) and high discharge capacity of 480 mAh g −1 with three wellpronounced plateaus in the first discharge curve. The charge capacity of 55 mAh g −1 corresponds to extraction of three sodium atoms per unit cell, what perfectly matches the results of quantitative EDX analysis, showing the Na:V = 6:14 atomic ratio for the material charged to 4.6 V ( Table 2). The plateau located around 1.6 V is sloping, in contrast to two more flat plateaus at 0.8 and 0.5 V. The absence of any plateau in the second and all further discharge cycles accompanied by a fast fading of specific capacity (Figure 5d) indicates the possible conversion mechanism with fast structural degradation. At the same time, one can notice the growing charge capacity up to third cycle followed by its fading on the next cycles. This means that at first cycle, the extra sodium atoms, incorporated into the crystal structure upon discharge, can be reversibly extracted upon charge. Starting from the fourth cycle, the structural degradation prevails and hinders the (de)intercalation process, which is clearly represented by gradual fading of discharge capacity vs. cycle number (Figure 5d). The situation is quite similar for the material cycled at C/5 rate (Figure 5c), but the plateaus at 0.8 and 0.5 V are not so evident here, indicating kinetic limitations. To minimize the effect of the high cell potential on structural degradation, we performed galvanostatic cycling in the anodic area ( Figure S4), but the resulting curves show the same tendency.

Electrochemical Characterization
As mentioned in the Materials and Methods section, the active material was ball-milled to reduce the particle size. However, the discharge capacity in the first discharge cycle was only ~32 mAh g −1 for the 1.5-4.8 V potential window ( Figure S3). The long plateau during the first charge above ~4.6 V is tentatively attributed to the electrolyte oxidation, thus further tests were limited by the 4.6 V threshold.
Galvanostatic cycling in the 0.1-4.6 V potential window was performed at C/20, C/10 and C/5 rates (Figure 5a-c). The shape of galvanostatic curves looks quite similar for the C/20 and C/10 rates and can be characterized by a low charge capacity in the first cycle (~50-60 mAh g −1 ) and high discharge capacity of 480 mAh g −1 with three well-pronounced plateaus in the first discharge curve. The charge capacity of 55 mAh g −1 corresponds to extraction of three sodium atoms per unit cell, what perfectly matches the results of quantitative EDX analysis, showing the Na:V=6:14 atomic ratio for the material charged to 4.6 V ( Table 2). The plateau located around 1.6 V is sloping, in contrast to two more flat plateaus at 0.8 and 0.5 V. The absence of any plateau in the second and all further discharge cycles accompanied by a fast fading of specific capacity (Figure 5d) indicates the possible conversion mechanism with fast structural degradation. At the same time, one can notice the growing charge capacity up to third cycle followed by its fading on the next cycles. This means that at first cycle, the extra sodium atoms, incorporated into the crystal structure upon discharge, can be reversibly extracted upon charge. Starting from the fourth cycle, the structural degradation prevails and hinders the (de)intercalation process, which is clearly represented by gradual fading of discharge capacity vs. cycle number (Figure 5d). The situation is quite similar for the material cycled at C/5 rate (Figure 5c), but the plateaus at 0.8 and 0.5 V are not so evident here, indicating kinetic limitations. To minimize the effect of the high cell potential on structural degradation, we performed galvanostatic cycling in the anodic area ( Figure S4), but the resulting curves show the same tendency.

Discussion
Additional information about the electrochemical processes involved during Na (de)intercalation is provided by dQ/dV curves at the C/20 rate (black curve in Figure 6a). The first cycle dQ/dV curve shows a broad cathodic peak at 3.3-3.4 V corresponding to the Na extraction, and an incomplete process of further Na extraction at 4.6 V. The first cathodic peak matches well with the (de)intercalation potential reported for γ-Na x V 2 O 5 [7,9]. Anodic dQ/dV curve at the first discharge reveals Na insertion at potentials of 0.85 V, 0.48 V and 0.25 V. The origin of a small broad peak near 1.5 V is ambiguous and can be tentatively interpreted as a minor amount of embedded Na. dQ/dV curves for further cycles confirm the irreversible character of the Na insertion during the first discharge and no anodic peaks are observed anymore. At the same time, the cathodic curves for second and third cycles reveal a sharp peak at 3.9 V which becomes broader and shifts towards 4.0 V and 4.2 V at fourth and fifth cycles, respectively. These well-pronounced cathodic peaks indicate quantitative sodium extraction from the structure formed upon the first discharge.  Figure S5). The difference of unit cell volumes for the pristine and 1-V-discharged compounds is about 0.6 Å 3 and falls into the range of two standard deviations, keeping in mind the unit cell volume of 1500 Å 3 ( Table  1). The absence of a significant change of the unit cell volume corresponds to a low discharge capacity of ~ 60 mAh g −1 registered at 1 V, since no significant amount of sodium has been intercalated into the structure at this potential. Moreover, [010] SAED pattern and HAADF-STEM image taken from the 1-V-discharged material ( Figure S6) are obviously identical to those of pristine Na9V14O35. Both SXRD and TEM data correlate with the dQ/dV curve in Figure 6a, showing that no considerable Na insertion occurs above 1 V.  Figure S5). The difference of unit cell volumes for the pristine and 1-V-discharged compounds is about 0.6 Å 3 and falls into the range of two standard deviations, keeping in mind the unit cell volume of 1500 Å 3 ( Table 1). The absence of a significant change of the unit cell volume corresponds to a low discharge capacity of 60 mAh g −1 registered at 1 V, since no significant amount of sodium has been intercalated into the structure at this potential. Moreover, [010] SAED pattern and HAADF-STEM image taken from the 1-V-discharged material ( Figure S6) are obviously identical to those of pristine Na 9 V 14 O 35 . Both SXRD and TEM data correlate with the dQ/dV curve in Figure 6a, showing that no considerable Na insertion occurs above 1 V. range of two standard deviations, keeping in mind the unit cell volume of 1500 Å ( Table  1). The absence of a significant change of the unit cell volume corresponds to a low discharge capacity of ~ 60 mAh g −1 registered at 1 V, since no significant amount of sodium has been intercalated into the structure at this potential. Moreover, [010] SAED pattern and HAADF-STEM image taken from the 1-V-discharged material ( Figure S6) are obviously identical to those of pristine Na9V14O35. Both SXRD and TEM data correlate with the dQ/dV curve in Figure 6a, showing that no considerable Na insertion occurs above 1 V.   Figure S7). The expansion of the structure along the aand c-axes is small (0.9% and 2.8%, respectively), but the expansion along the b-axis is huge and amounts to~25.2%. This difference reflects the rigidity of the (010) layers formed by tightly interlinked VO 5 square pyramids and VO 4 tetrahedra, in which the expansion in the a-c plane occurs through elongation of the V-O bonds upon reduction of vanadium cations with increase in their ionic radius (r(V 2+ ) = 0.79 Å, r(V 3+ ) = 0.64 Å, r(V 4+ ) = 0.58 Å, r(V 5+ ) = 0.54 Å, CN = 6) [24]. Large increase in the interlayer separation is in line with the necessity to provide enough space to accommodate large amounts of Na, as the specific capacity at 0.25 V exceeds 400 mAh g −1 . This must cause structure instability, and indeed, the crystals with another symmetry were found in the 0.25 V-discharged material, as one can see from the SAED patterns and high-resolution HAADF-STEM images, which are typical for a disordered rock-salt (DRS) structure with F-centered cubic lattice, a unit cell parameter a~4.7 Å ( Figure 8) and a Na:V ≈ 1:1 atomic ratio ( Table 2). This observation is in agreement with the formation of the DRS structure, in which Na and V atoms randomly occupy the same crystallographic positions. The formation of the DRS structure was observed earlier in the related Li-ion system [18], in which it demonstrated a stable cycling at anodic potentials. However, the conversion process continues further in Na 9 V 14 O 35 up to 0.1 V resulting in a complete amorphization as indicated by absence of any reflections in the corresponding SXRD pattern (Figure 7, turquoise curve). Regarding the Na and V distribution, the 0.1-V-discharged sample is strongly inhomogeneous. It still contains particles with the Na:V ≈ 1:1 atomic ratio (Table 2), but another phase, strongly enriched with Na up to Na:V ≈ 6.7:1 (Table 2, Figure S8), also appears in the sample, and is probably responsible for the high capacity of 490 mAh g −1 . The multicomponent nature of active material at 0.1 V and progressing structural degradation upon further cycles cause the fast decrease of discharge capacity even at low current density (Figure 5d). served earlier in the related Li-ion system [18], in which it demonstrated a stable cycling at anodic potentials. However, the conversion process continues further in Na9V14O35 up to 0.1 V resulting in a complete amorphization as indicated by absence of any reflections in the corresponding SXRD pattern (Figure 7, turquoise curve). Regarding the Na and V distribution, the 0.1-V-discharged sample is strongly inhomogeneous. It still contains particles with the Na:V ≈ 1:1 atomic ratio ( Table 2), but another phase, strongly enriched with Na up to Na:V ≈ 6.7:1 (Table 2, Figure S8), also appears in the sample, and is probably responsible for the high capacity of 490 mAh g −1 . The multicomponent nature of active material at 0.1 V and progressing structural degradation upon further cycles cause the fast decrease of discharge capacity even at low current density (Figure 5d). To draw the correlation between the structural transformation and change in the oxidation state of vanadium upon electrochemical cycling, we recorded EELS spectra in the vicinity of V-L 3,2 edge ( Figure 9). The spectra were interpreted in terms of correlation between the vanadium oxidation state and the onset of the V-L 3 edge as proposed by Tan et al. [25]. The empirical dependence between the V-L 3 edge onset E V determined at 10% of the maximum height of the V-L 3 edge, and the vanadium formal oxidation V V state demonstrates a linear increase of E V with V V . We have used the linear E V -V V equation derived from a set of standard materials by Tan et al. [25] to estimate the vanadium oxidation state in the samples under investigation. In the pristine material, the onset energy is at E V = 515.2 eV that corresponds to V V = +4.3, in good agreement with the average oxidation state of +4.36 from the chemical composition Na 9 V 4.1+ 10 O 19 (V 5+ O 4 ) 4 . Charging to 4.6 V vs. Na + /Na slightly shifts the V-L 3,2 edge towards a higher energy loss resulting in E V = 515.4 eV and V V = +4.5 that corresponds to extraction of 3 Na per Na 9 V 14 O 35 formula unit as deduced from the electrochemical data and EDX analysis (Na 6 V 14 O 35 , V V = +4.57). Upon discharge to 1 V and 0.25 V, the V-L 3 edge onset energy is reduced to E V = 514.8 and 514.3 eV, respectively, providing V V = +3.9 and +3.5. It should be noted that the vanadium reduction from +4.5 to +3.5 upon discharge from 4.6 V to 0.25 V accounts for only~250 mAh g −1 capacity that corresponds to the end of the first discharge plateau. Thus, the capacity of the second and third discharge plateaus should be attributed to a conversion reaction with the formation of the Na-rich phase. Unfortunately, the two-phase nature of the sample discharged to 0.1 V and strong Na inhomogeneity even within the same crystallite ( Figure S8) prevent sensible EELS measurement of V V . eV, respectively, providing VV = +3.9 and +3.5. It should be noted that the vanadium reduction from +4.5 to +3.5 upon discharge from 4.6 V to 0.25 V accounts for only ~250 mAh g −1 capacity that corresponds to the end of the first discharge plateau. Thus, the capacity of the second and third discharge plateaus should be attributed to a conversion reaction with the formation of the Na-rich phase. Unfortunately, the two-phase nature of the sample discharged to 0.1 V and strong Na inhomogeneity even within the same crystallite ( Figure S8) prevent sensible EELS measurement of VV.

Synthesis
Na9V14O35 contains vanadium in two oxidation states, 4+ and 5+. The synthesis in vacuum-sealed silica ampoules was implemented to stabilize vanadium in the intermediate oxidation state as it was previously reported by Millet at al. [22,23]. Initially, NaVO3 was synthesized by annealing the mixture of stoichiometric amounts of Na2CO3 (Ruskhim, 99%) and V2O5 (Sigma Aldrich, 99.6%) at 550 °C for 14 h. Single-phase polycrystalline Na9V14O35 was prepared from 3.06 mmol NaVO3, 0.05 mmol V2O5 and 0.8 mmol V2O3 (Alfa Aesar, 99.7%). The mixture of the initial reagents with the total weight of 0.5 g was ground thoroughly in an agate mortar, pressed into a 10 mm pellet, placed into an alumina crucible and sealed into a 16 mm quartz tube under dynamic vacuum of ~5•10 −3

Synthesis
Na 9 V 14 O 35 contains vanadium in two oxidation states, 4+ and 5+. The synthesis in vacuum-sealed silica ampoules was implemented to stabilize vanadium in the intermediate oxidation state as it was previously reported by Millet at al. [22,23]. Initially, NaVO 3 was synthesized by annealing the mixture of stoichiometric amounts of Na 2 CO 3 (Ruskhim, 99%) and V 2 O 5 (Sigma Aldrich, 99.6%) at 550 • C for 14 h. Single-phase polycrystalline Na 9 V 14 O 35 was prepared from 3.06 mmol NaVO 3 , 0.05 mmol V 2 O 5 and 0.8 mmol V 2 O 3 (Alfa Aesar, 99.7%). The mixture of the initial reagents with the total weight of 0.5 g was ground thoroughly in an agate mortar, pressed into a 10 mm pellet, placed into an alumina crucible and sealed into a 16 mm quartz tube under dynamic vacuum of~5·10 −3 mbar. The tube was heated with 120 • C/h heating rate to 650 • C, annealed for 50 h and cooled down with the furnace. The pellet was crushed, reground, pressed into a pellet and annealed at the same conditions for the second time.

Cells Assembling and Electrochemical Testing
Electrode composite was prepared from 80 wt.% Na 9 V 14 O 35 , 10 wt.% carbon black (Super P) as a conductive additive and 10 wt.% polyvinylidenefluoride (PVDF) as a binder. Prior to preparation of a slurry, active material mixed with carbon black and moistened with acetone was subjected to 1 h treatment in a high-energy SPEX-8000 ball mill (SPEX CertiPrep, Metuchen, NJ, USA) to reduce particle size and perform an effective mixing of all components. Then, PVDF was dissolved in N-methyl-2-pyrrolidone and mixed with the active material into slurry. The resulting slurry was coated on an aluminum foil with the Dr. Blade applicator and then dried in vacuum at 85 • C. Next, 16 mm circular electrodes made from the coated foil were dried in vacuum at 110 • C overnight.
The electrochemical cells were assembled in an argon-filled MBraun glove box (Mbraun, Garching, Germany)) with the residual oxygen and water content less than 0.1 ppm. A typical cell for galvanostatic cycling consisted of an electrode with the active material, a borosilicate glass fiber separator and sodium metal used as the counter electrode. Then, 1 M solution of NaClO 4 in sulfolane:propylene carbonate mixed in 1:1 volume ratio was used as the electrolyte. Galvanostatic cycling with potential limitation was performed on a BioLogic potentiostat (BioLogic, Seyssinet-Pariset, France) at C/20, C/10 and C/5 rates, where 1C corresponds to the current density of 163 mA g −1 (removal/insertion of all sodium from/into the formula unit within one hour).

Scanning and Transmission Electron Microscopy
The morphology was characterized with a Quattro S ESEM scanning electron microscope (ThermoFisherScientific, Landsmeer, Netherlands). Samples for transmission electron microscopy were prepared by crushing the Na 9 V 14 O 35 powder in an agate mortar in acetone followed by depositing the suspension onto holey TEM grid with a Lacey/Carbonsupporting layer. The electrodes at different state of charge were removed from electrochemical cells, double-washed in dimethyl carbonate (DMC) and scratched from the Al foil. All manipulations were performed in Ar-filled glove box followed by transportation to the TEM column by means of a dedicated vacuum holder, completely avoiding contact with air and moisture. Selected area electron diffraction (SAED) patterns, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray (EDX) compositional maps were taken with an aberration-corrected FEI Titan G3 transmission electron microscope (ThermoFisherScientific, Landsmeer, Netherlands) operated at 200 kV. The electron energy loss spectra (EELS) were registered in a STEM mode on a Titan Themis Z TEM (ThermoFisherScientific, Landsmeer, Netherlands) equipped with a Gatan Quantum ERS/966 P spectrometer (Gatan, München, Germany) and operated at 200 kV. Energy dispersion of 0.025 eV per channel was used; the energy resolution measured by full width at half maximum of zero-loss peak was 0.125 eV.

Conclusions
Na 9 V 14 O 35 (η-Na x V 2 O 5 ) has been synthesized by a solid-state route in an evacuated sealed silica tube and tested as electroactive material for Na half-cells. Being charged to 4.6 V vs. Na + /Na, almost 3 Na can be extracted per Na 9 V 14 O 35 formula unit, resulting in a charge capacity of about 60 mAh g −1 . Upon discharge below 1 V, Na 9 V 14 O 35 uptakes Na up to the Na:V = 1:1 atomic ratio that is accompanied by a drastic increase of the separation between the layers of the VO 4 tetrahedra and VO 5 tetragonal pyramids, and a volume increase of about 31%. The induced structure instability triggers a transformation of the ordered layered Na 9 V 14 O 35 structure into a rock-salt type disordered structure. Ultimately, the amorphous products of a conversion reaction are formed at 0.1 V, delivering the discharge capacity up to 490 mAh g −1 , which, however, quickly fades with the number of charge-discharge cycles.
Supplementary Materials: The following are available online, Figure S1: Experimental and calculated PXRD profiles (and their difference) after Rietveld refinement of Na 9 V 14 O 35 . The black ticks indicate the Bragg reflection positions, Figure S2: HAADF-STEM image, individual Na, V and mixed Na/V EDX maps of the pristine Na 9 V 14 O 35 material; Figure S3: Initial charge-discharge curves of the ball-milled Na 9 V 14 O 35 sample cycled with a rate of 16.3 mA g −1 (corresponding to C/10 as it is defined in the main text) (potential window 1.5-4.8 V vs. Na + /Na); Figure S4: Galvanostatic cycling for Na 9 V 14 O 35 in Na cell in a potential window of 0.1-3.0 V vs. Na + /Na (a) and comparison of specific capacity in the different potential windows at C/10 rate (b); Figure S5: Experimental and calculated profiles (and their difference) after a Le Bail analysis of the SXRD pattern of Na 9 V 14 O 35 discharged to 1 V vs. Na + /Na. The black ticks indicate the Bragg reflection positions of the P2/c unit cell. Wavelength λ = 0.20736 Å; Figure S6: [010] SAED patterns of pristine (a) and discharged to 1 V (c) Na 9 V 14 O 35 . High-resolution [010] HAADF-STEM images of pristine (b) and discharged to 1 V (d) Na 9 V 14 O 35 ; Figure S7: Experimental and calculated profiles (and their difference) after Le Bail analysis of the SXRD pattern of Na 9 V 14 O 35 discharged to 0.25 V vs. Na + /Na. The black ticks indicate the Bragg reflection positions of the P2/c unit cell. Wavelength λ = 0.20736 Å; Figure S8: HAADF-STEM images, individual Na, V and mixed Na/V EDX maps of Na 9 V 14 O 35 discharged to 0.1 V, Table S1: Fractional atomic coordinates and occupancies for pristine Na 9 V 14 O 35 , Table S2: Selected interatomic distances for pristine Na 9 V 14 O 35 (Å).